This invention relates to an electrographic printer having a paper path near the top of the printer, a small diameter photoconductive drum and a charged transfer belt associated with the drum. The charged transfer belt is positioned opposite the photoconductive drum and functions to transfer a developed image from the photoconductive drum to a sheet of paper. The charged transfer belt also functions to transport the imprinted sheet of paper to a fusing station. The drum is seamless and therefore printing may begin at any position on the drum. The electrographic printer of this invention is particularly suitable for continuous printing as in a desktop publishing environment.
In the process of electrographic or xerographic printing, a photoconductive member is employed to record an image. The photoconductive member, which may be in the form of a belt or a drum, is charged to a substantially uniform potential to sensitize its photosensitive surface. In the case of a copying machine, a light is shined on an original document to be copied. Through the use of lenses, mirrors, and various other optical components, the charged portion of the photoconductive surface is exposed to a reflected light image of an original document to be reproduced. The light image is recorded as an electrostatic latent image on the photoconductive member. This latent image corresponds to the informational areas contained on the original document.
In the case of an electrographic printer connected to a computer, a similar process is used to record information on the photoconductive member. The charged portion of the photoconductive surface is exposed to a light image produced by an optical print head. The precise shape of the light image is controlled by input signals from the computer. For example, a laser or an LED array may be used as an optical print head which receives input signals from the computer to illuminate the photoconductive member with a light image of a particular shape. Here too, an electrostatic latent image corresponding to the desired informational areas is recorded on the photoconductive member.
After recording the electrostatic latent image on the photoconductive member, the latent image is developed by bringing a developer material or toner into contact with it. The developer material is attracted to the electrostatic latent image and forms a developed or powder image on the photoconductive member corresponding to the electrostatic latent image. The powder image is subsequently transferred to a sheet of recording medium, such as a sheet of paper, in a transfer region. Thereafter, the powder image is permanently affixed to this sheet in image configuration by a variety of methods, such as by fusing.
The above-mentioned operations may be carried out by arranging a number of stations in sequence about the photoconductive member. Thus, the photoconductive member is usually surrounded in sequence by a charging station to charge the photoconductive member, an imaging station to form an electrostatic latent image on the photoconductive member, a developing station to develop the electrostatic latent image on the photoconductive member, and a transfer station to transfer the developed image from the photoconductive member to the sheet of recording medium. A discharging station and a cleaning station are also arranged about the photoconductive member to ready it for use again.
The assignee of the present application has filed a number of patent applications which disclose and claim an electrographic printing apparatus, and various components thereof, operating in accordance with the aforementioned principles. The electrographic printing apparatus is disclosed in U.S. Pat. No. 4,664,507, issued May 12, 1987. The electrographic printer/copier described in that patent employs a photoconductive belt assembly in the form of a disposable cassette. The photoconductive belt assembly is disclosed in U.S. Pat. No. 4,657,369, issued Apr. 14, 1987. The printer/copier described in U.S. Pat. No. 4,664,507 also employs the combined developing and cleaning unit which is the subject of U.S. Pat. No. 4,639,116, issued Jan. 27, 1987. All of the aforementioned patents are assigned to the present assignee and all are incorporated herein by reference.
The electrographic printer/copier described in U.S. Pat. No. 4,664,507 has a simplified paper path permitting access from the top of the machine. The paper path is substantially planar and is located directly below the top of the machine. In that electrographic printer/copier, the cassette containing the photoconductive belt is mounted vertically within the machine and a latent image is developed on the underside of the copy sheet as it passes over and comes in contact with the top of the photoconductive belt assembly.
The electrographic printer/copier described in that patent requires two rotations of the photoconductive belt per copy produced. In actual practice, it is capable of producing about 12 copies per minute. During the first rotation of the photoconductive belt, the belt is uniformly charged and a latent image is generated by means of an optical print head on the surface of the photoconductive belt. The latent image thus formed is developed by the deposition of toner from a combined developer/cleaning unit operating in the develop mode. The belt then enters the transfer region wherein the developed image is transferred to the underside of the paper or other copy material. In the transfer region, a transfer unit generates an electrical field which attracts the toner from the photoconductive belt to the underside of the paper. This completes the first rotation of the belt as the paper is transported to a fusing station by means of a vacuum transport unit attached to the lid of the machine. The vacuum transport unit is in contact with the upper side of the paper only. Smudging is thereby avoided since the developed image is on the underside of the paper.
During the next revolution of the belt, the belt is prepared for making the next copy. The main charging unit and the optical print head are disabled while an erase lamp is activated and the developer/cleaner unit is switched to the clean mode. Thus, as the belt continues to rotate following image transfer, the photoconductive belt is discharged by an erase lamp and the excess toner is removed using a conventional electrostatic process by the developer/cleaner unit. The belt is thereby readied for printing on the next page.
One of the primary objects of the electrographic printer/copier described in U.S. Pat. No. 4,664,507 is to provide a machine which is reliable over an extended period of use and which is readily serviceable. To accomplish this, the machine is provided with modular units which are easily removed and replaced at specified time intervals or when they malfunction. In addition, the machine is provided with a simplified paper path whereby the paper always travels along a substantially planar path located near the top of the machine. The paper is imprinted on its underside as it travels along this paper path and passes across the top of the vertically mounted photoconductive belt.
By providing this "straight-through" paper path and "top transfer" geometry, the number of paper jams is considerably reduced. In the event a paper jam does occur, the lid of the machine can be opened and the jammed paper can be readily accessed from the top of the machine. This is in contrast to many prior art machines wherein paper jams can only be reached from the side of the machine. Additionally, because the paper is imprinted on its underside, and because the machine has a "straight-through" paper path, the paper is ejected into the output tray face down. Thus, the paper is automatically collated as it is being printed and ejected.
The present invention is similar in many respects to the electrographic printer/copier described in U.S. Pat. No. 4,664,507. It includes many of the modular components which are employed in the electrographic printer/copier of U.S. Pat. No. 4,664,507. It also includes a paper path located near the top of the machine which permits easy access from the top of the machine in case of paper jams However, the machine of the present invention is also different from the electrographic printer/copier of U.S. Pat. No. 4,664,507 in many respects as will be described hereinbelow.
The machine of the present invention is more specifically directed to an electrographic printer interfaced to a computer. With suitable changes, the device described herein can be modified so that it functions as an electrographic copying machine. However, it is most suitably adapted for printing in contrast to copying, and in particular for continuous printing. Continuous printing is accomplished by employing a seamless photoconductive drum of relatively small diameter, and by employing suitable software which controls operation of the optical print head. Since the drum is seamless, there are no predefined start/stop points on the drum and printing can begin anywhere on the drum. Thus, there is no wasted rotation of the drum. Moreover, the relatively small diameter of the drum permits it to be rotated at relatively high speed.
The machine of the present invention further includes a charged transfer belt which is aligned opposite the photoconductive drum. The charged transfer belt has a dual function. First, it creates an electric field which attracts the developed image from the photoconductive drum to the underside of a sheet of paper brought into the transfer zone. Second, the static charge on the belt retains the sheet against itself and the belt transports the sheet from the photoconductive belt to a fusing station. The combination of the top paper path, the alignment of the charged transfer belt, and the small diameter drum is such that the sheet enters and leaves the transfer zone between the drum and the transfer belt at relatively sharp angles. The transfer zone is relatively short compared to prior art electrographic printers/copiers.
Despite these differences, the electrographic printer of the present invention still resembles the printer/copier described in U.S. Pat. No. 4,664,507 in that it includes a simplified paper path located near the top lid of the machine. It also employs the same top transfer geometry of the printer/copier of U.S. Pat. No. 4,664,507 whereby the sheet of paper is imprinted on its underside as it passes across the top of the photoconductive member.